1. Field of the Invention
The present invention relates to a semiconductor light emitting element. More particularly, the present invention relates to a structure of, and a method for fabricating, a quadruple alloy light emitting diode (LED) made of a quadruple alloy material of AlGaInP for constituting a high-luminescence LED which emits light in a red to green band.
2. Description of the Related Art
In recent years, a high-luminance quadruple alloy LED made of AlGaInP has become the object of particular attention as a light emitting element for various types of display devices for indoor use and outdoor use. A quadruple alloy material allows for the fabrication of an LED which emits light in a wide visible wavelength region ranging from a red to green band.
A typical structure of a conventional quadruple alloy LED 1100 for a yellow band is shown in FIG. 7A and 7B: FIG. 7A is a perspective view thereof; and FIG. 7B is a schematic cross-sectional view thereof.
In this structure, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 51 (doped with Si, carrier concentration: about 5.times.10.sup.17 cm.sup.-3, thickness: about 1.5 .mu.m), a non-doped (Al.sub.0.3 Ga.sub.0.7).sub.0.5 In.sub.0.5 P active layer 52 (thickness: about 0.7 .mu.m), a p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 53 (doped with Zn, carrier concentration: about 5.times.10.sub.17 cm.sup.-3, thickness: about 1.5 .mu.m), a p-Al.sub.0.7 Ga.sub.0.3 As current diffusion layer 54 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 5 .mu.m), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 0.5 .mu.m) are sequentially formed in this order on an n-GaAs substrate 50 by metal organic chemical vapor deposition (MOCVD). In addition, lower and upper electrodes 56 and 57 are formed on the reverse surface of the substrate 50 and on the top surface of the grown layered structure, respectively. The upper electrode 57 on the top surface of the grown layered structure, as well as the p-GaAs ohmic contact layer 55, have been patterned so as to be in a circular shape in the center region of the top surface of the structure. Portions of the upper electrode 57 and the p-GaAs ohmic contact layer 55 have been removed by performing an etching, leaving the circular-shaped portions remaining in the center region.
An axial luminous intensity (unit: candela (cd)) of a molded LED element is one of the indices representing the luminescence of the LED. In the conventional LED 1100 shown in FIGS. 7A and 7B, when the axial spread angle of the emitted light is about .+-.4 degrees with an operating voltage of about 2.0 V and a drive current of about 20 mA, the axial luminous intensity is about 8 cd.
The apparent axial luminous intensity is increased as the light concentration characteristics of an LED are improved (i.e., as the axial spread range of the emitted light is smaller). Moreover, an LED having improved light concentration characteristics can be advantageously used for communication applications.
Another conventional LED 1200, for communication, is shown in FIGS. 8A and 8B: FIG. 8A is a perspective view thereof; and FIG. 8B is a schematic cross-sectional view taken along the line 8B-8B' of the LED 1200 shown in FIG. 8A. The conventional LED 1200 shown in FIGS. 8A and 8B is an AlGaInP alloy system LED for a yellow band and has the following structure.
As shown in the schematic cross-sectional view in FIG. 8B, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 51 (doped with Si, carrier concentration: about 1.times.10.sup.18 cm.sup.-3, thickness: about 1.0 .mu.m), a non-doped (Al.sub.0.3 Ga.sub.0.7).sub.0.5 In.sub.0.5 P active layer 52 (thickness: about 0.6 .mu.m), a p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 53 (doped with Zn, carrier concentration: about 1.times.10.sup.18 cm.sup.-3, thickness: about 1.0 .mu.m), an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P current constriction layer 58 (doped with Si, carrier concentration: about 2.times.10.sup.18 cm.sup.-3, thickness: about 0.4 .mu.m), a p-Al.sub.0.7 Ga.sub.0.3 As current diffusion layer 54 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 6 .mu.m), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 0.5 .mu.m) are sequentially formed in this order on an n-GaAs substrate 50 by MOCVD.
The center region of the n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P current constriction layer 58 has been etched away in a circular shape to form a light emitting region, and the p-Al.sub.0.7 Ga.sub.0.3 As current diffusion layer 54 is re-grown over the current constriction layer 58 including the etched and removed center region thereof. The reference numeral 59 denotes the re-growth interface.
In addition, lower and upper electrodes 56 and 57 are formed on the reverse surface of the substrate 50 and on the top surface of the grown layered structure, respectively. The upper electrode 57 and the p-GaAs ohmic contact layer 55 are formed in a doughnut shape in which the center regions thereof are etched away so as to have openings of the same size and shape as those of the etched and removed region of the current constriction layer 58.
In this conventional LED element 1200, an injected current flows in a concentrated manner into the center region, so that the reduced spot size of emitted light can be realized. As a result, the light concentration characteristics of the resulting element, which has been molded with a resin, can be improved and the axial luminous intensity thereof can be increased.
However, in the conventional LED 1200 shown in FIGS. 8A and 8B, the p-Al.sub.0.7 Ga.sub.0.3 As current diffusion layer 54 is re-grown on the underlying p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 53 containing Al. Thus, oxygen is likely to be absorbed into the re-growth interface 59 (see FIG. 8B), resulting in various losses such as adversely increased resistance and non-radiative recombination of injected carriers.
The typical operational characteristics of such a conventional LED 1200 are as follows: the axial spread angle is about .+-.2 degrees and the luminescence is about 16 cd with the operating voltage of about 3.0 V when a power of about 20 mA is supplied thereto. As compared with the conventional LED 1100 shown in FIGS. 7A and 7B (which is made of the same quadruple alloy material and has the axial spread angle of about .+-.4 degrees and the luminescence of about 8 cd with the operating voltage of about 2.0 V when a power of about 20 mA is supplied thereto), the axial luminous intensity of the LED 1200 shown in FIGS. 8A and 8B is increased only by as little as twofold while the operating voltage is considerably increased. In the element 1200 shown in FIGS. 8A and 8B, the luminescence has been expected to be increased fourfold (i.e., about 32 cd) since the axial spread angle thereof is decreased to about 1/2 of that of the element 1100 shown in FIGS. 7A and 7B.
In order to solve such problems as set forth above, another conventional semiconductor light emitting element 1300 having such a structure as that shown in FIG. 9 has been suggested. The shape of the current constriction layer and the electrode on the top surface of the grown layered structure of the semiconductor light emitting element 1300 shown in FIG. 9 is the same as that of the element 1200 shown in FIGS. 8A and 8B.
As shown in the schematic cross-sectional view in FIG. 9, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 51 (doped with Si, carrier concentration: about 1.times.10.sup.18 cm.sup.-3, thickness: about 1.0 .mu.m), a non-doped (Al.sub.0.3 Ga.sub.0.7).sub.0.5 In.sub.0.5 P active layer 52 (thickness: about 0.6 .mu.m), and a p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 53 (doped with Zn, carrier concentration: about 1.times.10.sup.18 cm.sup.-3, thickness: about 1.0 .mu.m) are sequentially formed in this order on an n-GaAs substrate 50 by MOCVD. Next, unlike the conventional element 1200 shown in FIGS. 8A and 8B, a p-GaInP layer 60 (doped with Zn, carrier concentration: about 1.times.10.sup.18 cm.sup.-3, thickness: about 100 .ANG.) not containing Al is formed on the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 53 in the element 1300 shown in FIG. 9. Since the layer 60 functions as the underlying layer during the regrowth process, oxygen is less likely to be absorbed into the re-growth interface 59, and conditions of the regrowth interface 59 can be improved as compared with the conventional example 1200 shown in FIGS. 8A and 8B.
The remaining part of the element 1300 shown in FIG. 9 is the same as that of the element 1200 shown in FIG. 8B. Specifically, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P current constriction layer 58 (doped with Si, carrier concentration: about 2.times.10.sub.18 cm.sup.-3, thickness: about 0.4 .mu.m), a p-Al.sub.0.7 Ga.sub.0.3 As current diffusion layer 54 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 6 .mu.m), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3.times.10.sup.18 cm.sup.-3, thickness: about 0.5 .mu.m) are sequentially formed in this order on the p-GaInP layer 60.